The present invention relates to an automatic transmission for use in a vehicle, and more particularly relates to a hydraulic fluid pressure control system for controlling the engagement of the gear speed stages of such a transmission.
Various automatic transmissions for use in vehicles are well known. Such an automatic transmission generally comprises a gear transmission mechanism which comprises a plurality of friction engaging mechanims, and one or another of a plurality of gear speed stages is achieved by selectively providing operating fluid pressures to various ones of the friction engaging mechanisms. Further, various hydraulic fluid pressure control systems are well known for controlling said selective supply of operating fluid pressures to the friction engaging mechanism. Such a hydraulic fluid pressure control system generally comprises: a source of hydraulic fluid pressure such as a pump; a line fluid pressure control valve which modifies the fluid pressure supplied from the fluid pressure source and produces a line fluid pressure which is regulated to an appropriate line fluid pressure level; a throttle fluid pressure control valve which modifies the line fluid pressure and produces a throttle fluid pressure which increases in accordance with increase of a parameter representative of the load upon the internal combustion engine of the vehicle--in most cases, in accordance with increase of throttle opening of the engine, which is taken as indicative of engine load; a governor fluid pressure control valve, which modifies the line fluid pressure and produces a governor fluid pressure which increases in accordance with increase of the velocity of the vehicle; a manual selector valve which is controlled by hand by the driver of the vehicle and which is selectable between a plurality of positions which correspond to desired ranges of gear speed stages, typically including a "D" range and at least one forward range which is not the "D" range; and a plurality of speed shift valves, which selectively shift supply of actuating fluid pressures to the plurality of friction engaging mechanisms, according to various equilibrium relationships between the throttle fluid pressure and the governor fluid pressure, in a per se well known fashion.
Thus, in such an automatic transmission, the desired transmission speed range is manually set by the driver on the aforesaid manual selector valve, and then, within this speed range, the various speed shift valves corresponding to speeds which are allowed to be selected within this speed range are shifted to and fro according to balance relationships between the opposing governor fluid pressure and throttle fluid pressure, in accordance with the operating conditions of the vehicle and of the internal combustion engine of the vehicle, and thus the gear transmission mechanism is set to a particular speed stage.
Further, it is known for the highest speed stage of such an automatic transmission to be constructed to be an overdrive speed stage. Often, this is attained by constructing the automatic transmission with an in line overdrive mechanism. Further, it is common for the speed stage that below the overdrive speed stage to be a directly connected speed stage. The operation of such a transmission in the overdrive speed stage is very advantageous when the vehicle is being driven on the open road, as on a freeway, for improving fuel consumption of the vehicle, for improving the drivability of the vehicle, and also for improving the quality of the exhaust emissions of the engine of the vehicle. However, difficulties have in the prior art arisen with regard to operation of the hydraulic fluid pressure control mechanism for such an overdrive speed stage. First, it is desirable that the overdrive speed stage should be engaged when the vehicle is being operated at low speed, because this sharply deteriorates the quality of the exhaust emissions of the engine of the vehicle. Further, when the vehicle is to be rapidly accelerated, the overdrive speed stage should not be engaged. Yet further, as a matter of course, when the vehicle is being decelerated by the use of engine braking, by the driver of the vehicle shifting the manual shift drive from "D" range to a lower forward range such as "3" range, it is necessary, if the automatic transmission is at that time operating in the overdrive speed stage, for the transmission to be compulsorily shifted down to the next speed stage below said overdrive speed stage, i.e. the directly connected speed stage, in order for said engine braking to be performed effectively. Further, if such an overdrive speed stage is engaged before the engine of the vehicle has been fully warmed up, problems again arise with regard to the quality of the exhaust emissions in the exhaust gases of the engine.
In the prior art, it has not been possible satisfactorily to provide a simple and cost effective solution to these various problems, which is reliable during use over a long period of time, and is easy to manufacture, and which does not render the automatic transmission unduly complicated, bulky, or heavy.